Correct Option: A

The black part of the moon is called sea of tranquility. Sea of tranquility is not an actual sea but rather the point at which Apollo 11 first landed on when it reached the moon. It is a lunar mare which mainly consists of basalt rock and is located on the Tranquillitatis basin which is on the Moon. The mare has a tint which is slightly blue in colour and stands out from the rest of the moon.